A: All sign up individually, 6 pairs of parents and 12 people: 12×400=4800, 6 children: 6×200=1200; add up = 6000; B: All group purchases, 12 parents + 6 children=18 people, 18×300
=5400C: Group purchase for 12 parents: 12×300=3600, half price for 6 children, 6×200=1200, total = 4800; so plan C is the best and most economical